cells of cystic PPBs from the normal small round blue cells (lymphocytes, etc.) occasionally present in the walls of CPAM type 1 or 4. Other cystic and pseudocystic lesions Peripheral cysts. Peripheral air-containing cysts of the lung can be seen in neonates, infants, young children and teenagers. They occur in association with Down’s syndrome, as a result of pulmonary infarction, or in association with idiopathic spontaneous pneumothorax. Occlusion of the pulmonary artery in infants can result in peripheral infarction of the lung, which, with necrosis and organisation, can produce subpleural cysts of varying sizes [22]. In Down’s syndrome, it has been suggested that the cysts are an intrinsic feature of the disease that may result from reduced post-natal production of peripheral small air passages and alveoli. The air-filled cysts are 0.2–1.0 cm in diameter and located beneath the pleura. They are formed of vascular fibrous connective tissue walls lined with alveolar lining cells [23]. Sequestrations. Sequestrations are classified as intralobar (often occurring in the older child or young adult and associated with repeated infections, the majority are thought to be acquired) and extralobar (most often occurring in neonates and young children and thought to be congenital), and both can have a cystic component [24]. In many of the former, cystic structures are formed by dilated airways containing mucopurulent material (fig. 12), whereas, in the latter, there is a strong association with CPAM type 2 within the lesion [10, 25, 26]. Congenital pulmonary lymphangiectasia. Congenital pulmonary lymphangiectasia is a rare, usually fatal, disorder involving both lungs and is characterised by distended lymphatics in the bronchovascular bundle, the interlobular septa and the subpleural space. A few cases of unilobar involvement have been described that were radiologically misdiagnosed as infantile (congenital) lobar emphysema [27]. The lymphatics can be identified immunohistochemically using one of the newer markers specific for lymphatic endothelium (D2-40 antibody or lymphatic vessel endothelial hyaluronan receptor 1). Fig. 12. – Intralobar sequestration. Gross picture of cut surface of resected specimen with a dominant mucus- filled cyst. J.T. STOCKER, A.N. HUSAIN 16